1. a) Field of the Invention
The present invention relates to a water-cooled plasma torch specially designed for use to drill into a refractory material.
The invention also relates to a method of using this specially designed plasma torch for opening a tap hole closed by a plug in a wall of a furnace, particularly a metal furnace, a blast furnace or an arc furnace.
2. Brief Description of the Prior Art
It is of conventional practice to provide one or more holes, called "tap holes", at the bottom of a blast furnace, an arc furnace or any other kind of metal furnace, in order to recover in a ladle the molten metal obtained after treatment within the furnace. The tap hole that is so provided in the wall of the metal furnace is usually of cylindrical form and used when the furnace is of rigid structure and cannot be tilted for pouring out the molten metal.
Of course, after each recovery of molten metal, it is necessary to stop the tap hole to avoid the slag on top of the liquid metal bath to pour out and mix with the metal in the ladle. This is usually done with a refractory plugging paste made of a mixture of clay, composed of alumina, silica and graphite. This plugging paste is extruded in the form of a roll which is inserted into the hole while it is extruded through a nozzle positioned in alignment with the hole to be stopped.
In order to achieve proper stopping of a tap hole, it is compulsory that the internal wall of the tap hole be properly calibrated so that the diameter of the plug-forming roll be identical to the diameter of the tap hole. Such a proper calibration involves that the opening of the tap hole after each operation of the furnace is made in a very precise manner.
Currently, there are four commonly used methods for opening tap holes in a furnace.
The first one consists in "drilling" the plug with a pneumatic drill. This method which is exclusively mechanical, has the advantages of not requiring water and drilling holes that are well calibrated. However, this first method has the drawback of being useful only when the tap holes are of limited depth, and only with moderate temperature. Moreover, the maintenance price of the equipment is very high because, in use, the bit at the end of the drill is usually destroyed or seriously damaged when it comes into contact with the molten metal.
The second method which is often used in combination with the first method disclosed hereinabove, consists in melting the plug with an oxygen lance. The lance which usually consists of a steel pipe, is inserted into the tap hole and a mixture of oxygen and iron powder is injected therethrough. The oxidation of the iron powder with the oxygen generates a substantial amount of heat which melts the plug. This second method is rather cheap but has the major drawbacks of resulting in a non uniform tapping because the reaction of iron with oxygen does not occur within the hole under a controlled fashion. As a result, cavities are generated in the wall of the tap hole, which substantially and negatively affects the calibration of it. Another drawback of this second method is that this lancing technique is almost impossible to automate.
The third method commonly used for opening tap holes, consists in melting the plug by means of an electric arc generated between a graphite electrode connected to the electrical supply of the furnace, and the plug itself. In practice, this third method is inefficient to open holes of a limited diameter especially when these holes are deep. Moreover, the material of the plug must be sufficiently conductive to permit arc transfer and the arc is never adequately directed As a result, the calibration of the hole is not respected.
The fourth method for opening tap holes consists in blasting the plugs with zinc balls. This fourth method is certainly well adapted to open tap holes in a very "clean" manner because the blasting effect is obtained by spontaneous vaporization of the zinc balls entering the tap hole. However the tap holes to be opened in this way must have a certain depth and be at high temperature before carrying out the blasting. In addition, there is a high risk of ricochets.
The uniformity and reproducibility of any tapping is important to keep the diameter of the tap hole well calibrated. The methods making use of an oxygen lance or an electrical arc transfer tend to enlarge the bottom of the hole because the heat is not directed and the dimension of the melting zone is not controlled. This in turn causes the new plug that is subsequently inserted, to be poorly held back and retained in the hole, thereby leading to major molten metal leaks that may be dangerous. It may also occur that the hole is so enlarged that it is not possible to stop the flow of metal with the plugging paste, thereby making it necessary to stop the furnace to repair the tap hole.
U.S. Pat. No. 4,289,949 assigned to SINTEF describes the above mentioned drawbacks of the commonly used methods for opening tap holes, and suggests to use a plasma burner to overcome the same. The plasma burner which is proposed in this patent, comprises a centrally arranged electrode and an outer electrode which are made of non-melting and subliming material constituting a continuously consumable system. The outer electrode is shaped as a pipe in which the other pipe or rod-shaped electrode is coaxially mounted. Means are provided to inject a gas through the annular passage between the electrodes up to the front ends of the electrodes where an electric discharge occurs from the end of the inner electrode.
The plasma burner disclosed in the SINTEF patent is principally designed for heating a metal melt. Although its application to open tap holes is disclosed in the specification, no example of such an application and test results are reported.
In practice, it is known that electrodes made of graphite or silicon carbide are particularly fragile and that the smallest knock on the electrodes can break the torch and make it inoperable.
It is also known that the use of two coaxial tubular electrodes does not permit to stretch the arc by increase of the gas flow, because the space between the electrodes remain the same and the working voltage is so low that the power transferred to the plasma gas is low too. Then, to obtain a significant power, the current must be very high and the erosion of the electrodes is important.
It is futher known that with two coaxial electrodes made of consumable material, it is not possible to obtain a plasma jet of well defined coaxial outline, although such is actually necessary to open tap holes in a precise and reproducible manner.
As a matter of fact, to the Applicant's knowledge, no plasma torch is currently available for use to open tap hole in a metal furnace. This is because none of the commercially available water-cooled torches is built for insertion into tap holes. Such an insertion of the torch inside the hole is necessary for melting plug to an important depth in order to reduce the drilling time and make it sure that the plug is molten in a precise manner. However, the majority of the commercially available torches cannot withstand the intense heat that exist within a tap hole. Accordingly, their insertion into a tap hole may result in water leak from their front sealing joints close to the fusion region.
Moreover, with the existing torches, there can be some deposit of molten material on the front electrode arc attachment point, causing irregular arc operation and shortening of the electrode life time.